Overload protection devices such as current limiters and positive temperature coefficient (PTC) conductive polymers, commonly called a PolySwitch.RTM. registered to Raychem and described in the U.S. Pat. No. 4,780,598, assigned to Raychem, and hereby incorporated by reference are usually used together as seen in FIG. 1, especially in a hazardous environment. Hazardous environments are occasions in which the atmosphere contains gas vapor or dust particles which could easily be ignited by sparks or thermal heating.
Individually, a PTC element has a relatively low resistance under the normal operating conditions for the circuit, but is "tripped", (i.e. converted into a high resistance state), when a fault condition such as an excessive current (short circuit) or temperature occurs. When the circuit protection device 29 comprising the PTC conductive polymers 10 is tripped, the current passing through the series connected PTC element 10 causes it to self-heat to an elevated temperature at which it is in a high resistance state. However, the leads 13 of the PTC circuit protection device 29 are usually made out of nickel, which does not provide enough resistance to limit the current through the battery 12 before the PTC element 10 increases its resistance to further limit the current and substantially provides an open circuit.
Individually, a current limiter 15 is, conventionally, a fixed resistor of some particular height, (such as a fixed series resistor which is a wire-wound or a nichrome strip or any other kind of a nichrome assembly as described in the H52910 Raychem sales brochure for the "Protection of Batteries with PolySwitch.RTM. Devices and hereby incorporated by reference) to provide a high enough resistance to limit the current. However, the current limiter, such as the commonly used nichrome strip, generally, allows a percentage of the maximum short circuit current to still flow under fault conditions, which may deliver enough energy to cause the fixed nichrome resistor or strip to overheat and cause ignition of the explosive atmosphere. Thus a PTC element is usually used in combination with the nichrome strip to prevent this occurrence as seen in FIG. 1 of an intrinsically safe battery pack.
However, the combination of both the nichrome strip and the PTC element increases both the size and the cost to the protection circuit that must be built inside the battery housing.